1. Field of the Invention
The present invention relates generally to telecommunications network management. More particularly, the present invention relates to reducing undesirable phase transients caused by switchover during restoration from equipment failure in a telecommunications network.
2. Related Art
Modern telecommunication equipment has given network designers greater flexibility to create complex and diverse network topologies. In part, this flexibility is achieved by allowing designers to control selector switches in network equipment. The selector switches provide network equipment with the capability to select from among several different sources of communications signal inputs or outputs.
In general, each piece of network equipment has both an input selector switch and an output selector switch. The input selector switch selects one input from a plurality of inputs. The one input can, for example, provide a source of synchronization for the equipment. The output selector switch selects one output from a plurality of outputs on which to transmit a signal. The output can, for example, provide a synchronization source for another piece of network equipment. By appropriately choosing network selector switch placements, a network designer can design a network topology for network synchronization. The topology must meet various engineering requirements including traceability and loop-free design.
The selector switches are generally remotely programmable. Thus, the network topology can be changed after a network designer chooses an initial configuration. Moreover, using equipments having programmable selector switches allows a network management system to restore a network following a failure to a configuration that continues to adhere to the engineering requirements. This process is known as "self-restoration." To perform such "self-restoration," the network controls replacement of a lost signal by setting the selector switch to a new position corresponding to a desired replacement signal. This process is known as "switchover." That is, the selector switch switches over from a position corresponding to the failed signal input to a position corresponding to the replacement signal input. In conventional systems, switchover is limited to switching between only two channels.
However, the "switchover" process often results in phase transients introduced into the signal being communicated. When the failed piece of equipment is repaired and put back into operation in the network, a second switchover occurs, wherein communication is routed through the repaired equipment, even though the temporarily used equipment is operational. This process is known as "reversion." Reversion occurs because the network sees the originally preferred signal has been placed back in the network. In conventional systems, the selector switch "reverts" back to the original setting.
However, reversion is generally undesirable when the communication signal is a synchronization signal. Where the signal is a synchronization signal, phase transients caused by switchover can lead to significant degradation in network performance. As a result, switchovers should be minimized to the extent possible. Network designers generally accept the risk associated with the switchover from the failed piece of network equipment. This is because the risk of performance degradation due to phase transients is acceptable relative to not having network access. However, reversion from the temporarily used equipment to the repaired equipment is not desirable where the two equipments are of substantially equal quality. Thus, what is required is a priority scheme for preventing undesirable switchover when a failed piece of equipment is repaired and placed back in a network. Moreover, the priority should provide sufficient flexibility to allow switching between more than two channels.